The present invention relates generally to practical thin film resistor structures and methods for integrating multiple thin film resistors of the same or different materials and/or different sheet resistances.
In the past, integrated circuit designers have been limited to one sheet resistance for the thin film resistors integrated into a particular chip. This has necessitated design compromises, especially in the case of laser trimmed high value resistors because in this case the resistors usually are quite narrow. Consequently, the minimum trimmable amount of resistance is a greater proportion of the total resistance, so the accuracy is substantially less for laser trimmed high value resistors unless their width is substantially increased. Also, if the single available sheet resistance is high but the designer needs to provide a low value precision resistor, then the resistor must be made very wide.
It is conventional to adjust the sheet resistance, and also the temperature coefficient of resistance (TCR), of a resistive thin film layer by using suitable thermal anneal cycles to achieve a target sheet resistance for a deposited NiCr or SiCr layer. Once the sheet resistance of a thin film layer is known, the amount of annealing needed to increase its sheet resistance and TCR to target values can be determined from empirical curves.
A typical substrate on which an integrated circuit thin film resistor is formed includes a silicon wafer on which a field oxide is formed. An intrinsic TEOS layer (tetra ethyl ortho silicate layer, which is the liquid precursor used to form the oxides) is formed on the field oxide. A BPTEOS (boron phosphorus doped oxide) layer is formed on the TEOS layer. Another TEOS layer is formed on the BPTEOS layer to keep the doping away from interconnect metallization. The doping referred to is used to lower the re-flow temperature, which improves planarization, and reduces the sharpness of the edges of the oxide steps in the structure. The doping also provides gettering of sodium to keep it and other contaminants away from the transistors previously formed in the silicon substrate. The doping also has been proven to be beneficial in mitigating field oxide threshold problems, thereby preventing parasitic MOS field devices from turning on. The doping also helps in preventing charge-spreading, wherein the field threshold voltage gradually degrades, causing the parasitic MOS field devices eventually to begin to turn on and degrade circuit performance.
Design engineers would be able to better optimize some integrated circuit designs if it were practical and economical to integrate thin film resistors of various sheet resistances into a single integrated circuit structure. However, there has been no practical, economical way to accomplish this because temperature processing cycles associated with forming subsequent thin film resistor layers after formation of a first thin film resistor layer would cause a variety of difficult integrated circuit processing problems. For example, controlling the effect of various thermal cycles on the sheet resistances and TCRs of the multiple thin film resistors formed on successive oxide layers may be very difficult. Also, the presence of metallization layers in integrated structures including thin film resistors on multiple layers may make it very difficult to design subsequent thermal cycles of the kind needed to be compatible with the thin film resistor properties.
U.S. Pat. No. 4,019,168 entitled “Bilayer of Thin Film Resistor and Method for Manufacture”, issued Apr. 19, 1977 to Franklyn M. Collins, describes an integrated circuit structure including a layer of tantalum on a layer of nichrome for the purpose of stabilizing the sheet resistance of the nichrome. However, the foregoing patent is not directed to issues regarding processing problems associated with forming multiple thin film resistors on different oxide layers in an integrated circuit structure.
Prior art structures that include interconnected polycrystalline silicon resistors and diffused resistors are well known. However, such structures including polycrystalline silicon resistors and diffused resistors do not meet many of the needs of modern integrated circuit design. Although it is highly desirable to provide a TCR value of zero for polycrystalline silicon resistors and diffused resistors, as a practical matter this is difficult to achieve. In contrast, it is relatively easy to achieve the TCR of zero in thin film resistors for most sheet resistances. Diffused resistors have high voltage coefficients, due to their associated voltage-dependent depletion regions which cause the resistance to change as a function of voltage applied across the diffused resistor. Also, high precision resistance values and precise ratio-matching are much more difficult to achieve for polycrystalline resistors and diffused resistors than is the case for thin film resistors.
There is an unmet need for a practical integrated circuit structure and method for providing different thin film resistors composed of different materials and/or of different sheet resistances, each thin film resistor being on a different oxide layer.
There also is an unmet need for a practical integrated circuit structure and method for providing different thin film resistors composed of different materials wherein the sheet resistance of one of the thin film resistors can be adjusted without unacceptably changing the sheet resistance of the other thin film resistor.